1. Field of the Invention
This invention relates to a process for removing impurities from acidic fluid solutions using a serpentine material and producing a reaction product therefrom comprising recoverable metal values and finely divided silica. More particularly, the invention relates to a process for removing impurities wherein the desired result is obtained by contacting the serpentine material with a weak acid such as H.sub.2 CO.sub.3 or H.sub.2 SO.sub.3. Even more specifically, the invention relates to such a purification process wherein an acid leach component is produced by contacting a waste industrial gas containing carbon dioxide and/or sulfur dioxide with water thereby reducing the air pollution which would otherwise result from the discharge of untreated waste industrial gases into the atmosphere.
2. Description of the Prior Art
A number of United States patent references were specifically discussed in the parent applications Ser. Nos. 66,878 filed Aug. 25, 1970, and 363,611, filed May 24, 1973, which applications have been specifically incorporated by reference into this application. In each instance, the reference was discussed and the differences between that reference and the present invention were outlined. Manifestly, the general state of the prior art is set forth in these references. Specifically, the references discussed in the parent applications are U.S. Ser. No. 717,864 to Jones; U.S. Pat. No. 1,226,731 to Westby; U.S. Pat. No. 2,386,337 to Moyer; U.S. Pat. No. 2,778,727 to Schaufelberger; U.S. Pat. No. 2,778,729 to Schaufelberger; U.S. Pat. No. 2,816,015 to Donaldson; U.S. Pat. No. 3,025,131 to Lerner; U.S. Pat. No. 3,085,861 to Thornhill; U.S. Pat. No. 3,243,262 to Carr et al.; U.S. Pat. No. 3,318,689 to Zubryckyj et al.; U.S. Pat. No. 3,338,667 to Pundsak; and U.S. Pat. No. 3,547,583 to Wilson.
Prior to discussing the prior art in detail, one problem with respect to a great deal of the prior art must be considered. That problem is the characterization of serpentine as a hydrous magnesium silicate as has been the case for many years. This has led to the reporting of the formula for serpentine as being Mg.sub.3 Si.sub.2 O.sub.7.2H.sub.2 O. This is a misnomer and an incorrect statement of the formula. Most of those authorities who have considered the question lately have generally answered that the serpentine formula is Mg.sub.6 (Si.sub.4 O.sub.10) (OH).sub.8. Some of these quthorities report the formula to be Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4. However, magnesium has a preferred coordination number of 6 with hydroxyl radicals; therefore, the unit mineral cell should contain six magnesium atoms.
The (Si.sub.4 O.sub.10) in the modern formula indicates a high order polymeric layer type anion which has a tendency to cause sheetlike and flaky minerals and several minerals with this anion decompose in acids to give a solid silica residue. Serpentine decomposes to solid silica with acid. (Si.sub.2 O.sub.7) is only dimeric (2 units of discrete anions) and minerals with this anion tend to form silicic acid gels with acids. Serpentine does not give silicic acid gel with acid. Serpentine has been shown to be in sheetlike layers of Mg(OH).sub.2 attached to other sheetlike layers of (Si.sub.4 O.sub.10) similar to kaolinite.
Furthermore, minerals with descrete anions like (Si.sub.2 O.sub.7) are characteristically hard, whereas serpentine is soft, indicating that it does not contain (Si.sub.2 O.sub.7).
The (OH).sub.8 in the modern formula indicates the presence of a powerful chemically active base. Powdered serpentine will almost immediately neutralize all acid and must contain a strong base. Serpentine on being heated to a high temperature of about 600.degree. C. yields water which must come from "structural water" consisting of (OH) groups. When such water is forced out the mineral cannot be reformed by the addition of water, which is further evidence of hydroxyl.
The (.2H.sub.2 O) in the older formula indicates the presence of water crystallization which are chemically neutral water molecules weakly coordinated to cations. They can be driven off the mineral at a low temperature of about 105.degree. C. (just above the boiling point of water). Once driven off they can be easily replaced and the mineral will return to its original nature. This is common with zeolites. They also produce a gel with acid.
Serpentine must be heated to about 600.degree. C. to drive off water. It then changes to olivene and silica and cannot be converted back to serpentine simply by adding water. The equation for this is ##STR1## Together with its quick acting base nature, this confirms that serpentine contains (OH) hydroxyl groups and not (.H.sub.2 O), water of crystallization.
Therefore, because of the presence of hydroxyl groups, serpentine should be classified as a hydroxide rather than as a hydrous magnesium silicate. Faulty classifications have caused a long misunderstanding of serpentine. The disclosure and emphasis in this patent application may assist in the development of many new uses for this abundant, unique substance in addition to the use to be discussed below.
Certainly, the beneficiation of serpentine ore materials is a well known and economically desirable process. Serpentine is decomposed specifically for the recovery of the magnesium metallic values therefrom and often the beneficiation process also results in the production of a finely divided silica material. Other, generally related mineral ores, such as garnierite, have also been subjected to beneficiating for the purpose of recovering metallic values. As is well known in the art, natural processes often cause ores, such as those mentioned above, to undergo a laterization process whereby the ores produced thereby are referred to as being laterities of the basic ore.
Specific discussions of laterities and processes for the beneficiation thereof are set forth in U.S. Pat. No. 3,318,689 to Zubryckyj et al., U.S. Pat. No. 3,244,513 to Zubryckyj et al. and in U.S. Pat. No. 3,146,091 to Green. As is known to the routinier in this art, laterities are residual weathered deposits produced as the result of long term weathering processes acting on primary serpentinites which generally constitute a mixture of serpentine, magnetite, sulfides and other minor materials. During the laterization process, the sulfides and magnetite are oxidized to hematite and limonitic iron minerals and the serpentine is often altered to a garnierite having a high nickel content. The deposits of the intimately mixed, nickel containing iron oxides often overlie the earthy and clayey garnierite material which, in turn, generally overlies the primary serpentinite. Laterite deposits form most easily under tropical conditions.
Although there are many such laterized deposits which are located in tropical countries, there are many more low-grade primary serpentinite deposits in the world which have not been subjected to the process of weathering, concentration of nickel and change in mineralization which characterize the laterization process.
As is clearly set forth in the Zubryckyj et al. and Green patents mentioned above, a prior heat treatment of the minerals, at temperatures in the order of 600.degree.-850.degree. C, is required. This is an extremely costly step which has the additional shortcoming of often causing serpentine materials to be converted into talc and olivine. Each of these latter materials is relatively insoluble as compared to serpentine and often is a complicating factor in the beneficiation thereof.
Moreover, the Zubryckyj et al. and Green processes require the use of expensive reducing agents which are used at the high temperatures so that dissolvable products may be produced. With respect to the dissolution step itself, the Zubryckyj et al. and Green processes each utilize sulfuric acid as the solvent. In this connection, it is to be noted that while Zubryckyj et al. suggest the use of sulfur dioxide in connection with the acid forming component, whenever sulfur dioxide is utilized, oxygen is also required and it is apparent that as a result, sulfur trioxide is produced; therefore, the acid component utilized in the Zubryckyj et al. process, is in fact sulfuric acid.
In the prior art processes discussed in the immediately preceding paragraphs, a relatively insoluble material is produced by the initial high temperature reductions required. Accordingly, leaching times of several hours are required even though in each process strong acids, such as sulfuric acid, are utilized.
In U.S. Pat. No. 2,788,729 to Schaufelberger, nickel and cobalt values are obtained from garnierite utilizing an acid leaching process. While Schaufelberger has recognized that sulfur dioxide does seem to have utility in connection with the beneficiation of garnierite, such utility becomes available only when the temperature and pressure conditions are such that the sulfur dioxide acts as a clathrate whereby the dissociation of the acid formed therefrom is increased to the point that the acid is capable of acting in the same manner as a strong acid. In this connection reference is made to Advanced Inorganic Chemistry, by Cotton and Wilkinson, John Wiley and Sons (1966), particularly at page 545.
In the first place, serpentine and garnierite are completely different materials with known and readily differentiable characteristics as shown in the following Table:
Secondly, although at first glance it seems that Schaufelberger U.S. Pat. No. 2,778,729 utilizes an acid material produced by dissolving sulfur dioxide in water to beneficiate garnierite, a complete reading of Schaufelberger's disclosure reveals that it is only when sulfur dioxide is utilized in combination with sufficiently elevated pressure and temperature conditions to create a clathrate, that the same can be utilized for beneficiating garnierite.
Many of the important characteristics and properties of the serpentine materials involved in the present application are discussed by Dana in his Manual of Minerology, John Wiley and Son (17th Ed. 1959), particularly at pp. 463 through 465.
TABLE ______________________________________ Characteristic Properties Serpentine Garnierite ______________________________________ Formula Composition Mg.sub.6 (Si.sub.4 O.sub.10)(OH).sub.8 (Ni, Mg) SiO.sub.3 . nH.sub.2 O Crystallography Prismatic, Monoclinic Amorphous, Earthy Fibrous, Crystals Incrustation Index of Refraction 1.49 - 1.57 1.59 Hardness 2 - 5 (usually 4) 2 - 3 Lustre Greasy, Silky Earthy, Dull Waxlike Color Variegated, Mottled Apple Green in light and Dark White Shades of Green Tests Decomposed by HCl Difficultly does not blacken Decomposed by HCl when heated in Blackens when closed tube heated in closed tube Varieties Bastite, Common, only Garnierite Retinalite, Bowenlite, Picrolite, Antigorite Marmolite, Chrysotile, Radiotine Origin A Deep Seated A surface alteration Mineral product ______________________________________
Dana indicates that serpentine is readily decomposed by the strong acid, hydrochloric acid. With respect to garnierite, Dana indicates that this material is decomposed only with difficulty by hydrochloric acid.
In accordance with the foregoing, it can be appreciated that the prior art processes for beneficiating serpentine ore materials all possess the common problem that strong acids and low pHs were required for beneficiating the serpentine ore material. Further, in many instances, prior to the present invention, it was believed by those of ordinary skill in this art that not only were strong acids required but also pretreatment of the ore and/or high temperatures and pressures during the dissolution process were absolutely requisite.
Turning now to the Wilson reference, it is disclosed that waste stack gases containing sulfur dioxide may be purified by a process in which the sulfur dioxide gas is entrapped by the metallic sulicates. Wilson suggests several available sources for the oxide and silicate materials including slag from reverberatory refining of copper pyritic type ores and "a great number of naturally occurring mineral substances." However, in each case Wilson specifically states that the starting material must be a mixture of metallic oxides and metallic silicates which are reactable with aqueous solutions of mineral acids to form salts and hydrates of silicic acid. Wilson's intent is to form either a gel containing chemically and physically combined sulfur dioxide or the system may be maintained in a liquid state by preventing dehydration of the dihydrated silicic acid.
All of the oxides described by Wilson as being suitable were previously known to be reactive against SO.sub.2 acid. Most of them are soluble in water and do not occur as natural minerals except rarely, MgO as periclase. Silica gel was also known to absorb sulphur compounds. Water is also known to absorb SO.sub.2 gas to form sulfurous acid. The conversion of dissolved SO.sub.2 gas to sulfur by reduction with H.sub.2 S is also well known.
Wilson's novelty (col. 2, line 21) lies in the use of a mixture of two classes of substances, reactive metallic oxides and metallic silicates or slag which can form a reactive gel with SO.sub.2 acid which can be controlled to provide either a gel or a solution from which sulfur is produced by reduction. It is this control which is central to the teachings of Wilson.
Wilson's use of a slag reagent effectively provides a reactive metallic oxide and a silicate which produces a gel which can be controlled. If he uses a mixture of metallic oxides and natural metallic silicates, the natural metallic silicates must be those that produce a gel with SO.sub.2 acid or his inventive control of the process is cancelled. A natural mineral silicate which does not produce a gel would be unsatisfactory because it would unnecessarily contribute only unwanted silicate ions or solid silica which would be detrimental because they would contribute an additional disposal problem.
For minerals to obtain simple ions like (SiO.sub.4) which are capable of forming gels with acids, they must be formed at very high temperatures, like Olivine, which is a high temperature igneous mineral (1910.degree. C.) and, therefore, does not contain such ions. Serpentine does not form such a gel.
Therefore, serpentine would be wholly unsatisfactory as Wilson's metallic silicate reagent. It is a type of silicate which does not produce a gel with sulfurous acid and would, at best, add unnecessary solid silica into Wilson's process. It is not a metallic oxide but an hydroxide.
Turning now to Wilson's use of a slag as a reactant in the process, slag is ordinarily produced by the combination (a mixture) of the gangue (silicate source) of the ore and the added flux (oxide source).
In copper refining slags, the gangue consists of silica and silicate minerals and the iron part of the sulfide ore minerals. The flux is usually calcium oxide. These are melted together at over 1100.degree. C. and finally discarded where they quickly solidify.
Molten slag is similar to a geologists magma. However, on the very long slow cooling of a magma, chemical bonds form and reform until the most stable combinations crystallize into minerals. When a slag is quick frozen in a matter of hours, a glass is formed in which there are no crystals, but, rather, a substance consisting of individual separate ions. If finely ground-up they are free to react without the breaking of stable bonds. It is a super-cooled liquid state with the ions in a metastable state. Practically all the substances found in slags are salts or orthosilicic acid (H.sub.2 SiO.sub.4) and metasilicic acid (H.sub.2 SiO.sub.3). Iron oxide gives a very fusible slag.
In the ionic theory of slag, the dominant ion in slag is (SiO.sub.4).sup.4-. The silica frameworks dissociate EQU 5SiO.sub.2 = (Si.sub.3 O.sub.10).sup.8- + 2Si.sup.4+
At higher temperatures the polymeric (Si.sub.3 O.sub.10).sup.8- further dissociates to more Si.sup.4+.
The basic oxides dissociate EQU CaO = Ca.sup.2+ + 0.sup.2-
and in the presence of the Si.sup.4+ EQU Si.sup.4+ + 40.sup.2- = (SiO.sub.4).sup.4-
o.sup.2- is a very powerful chemical base and slag owes its basicity to this ion. When slag is ground into a fine powder, contact with this ion becomes possible in its metastable condition. Because of the available O.sup.2-, Wilson's use of slag as a reactant is a good idea to neutralize SO.sub.2 acid. Also, because of the (SiO.sub.4).sup.4- present, a gel can be formed in SO.sub.2 acid which apparently collects SO.sub.2 substances. Slag is a good reactant also, because it is a cheap waste product. Negatively, it is hard and must be ground finely.
Conversely, serpentine is not slag-like, it is soft rather than hard. It contains no (SiO.sub.4) groups. Serpentine cannot form a gel in contact with an acid. Serpentine contains no O.sup.2- ion and cannot replace slag in Wilson's process.